Molecular and Cellular Signaling - Martin Beckerman

522 21. Learning and Memory

FIGURE 21.7. Signaling in the learning pathway in Aplysia leading to shortand longterm facilitation: Serotonin stimulates an increased production of cAMP by adenylyl cyclase through actions of GPCRs and Ga subunits.A transient facilitation of the signal response is produced by phosphorylation of ion channels by the catalytic subunit (PKAC) of protein kinase A. This modification alters the biophysical properties of the ion channel. Long-term responses are elicited by sustained serotonin signaling resulting in gene transcription and morphological changes at the synaptic terminal.

efficiency of the transmission process depends on both presynaptic and postsynaptic factors. First, it depends on the amount of neurotransmitter release from the presynaptic terminal for a given depolarization or action potential. Second, it depends on how strong a response that release elicits at the postsynaptic side. The strength of the presynaptic signal and the postsynaptic response depends on the mix of ion channels and signal receptors and on their biophysical states.

In the case of short-term facilitation in Aplysia, the increased efficiency in synaptic transmission is implemented at the presynaptic side of the cleft. Calcium influx resulting from membrane depolarization and serotonin induced G protein signals converge on adenylyl cyclase, which responds by increasing cAMP production leading to changes in the amount of neurotransmitter being released from the terminal. Adenylyl cyclase acts as a signal integrator, serving as a moleculer device for detecting the temporal coincidence of electric shock and weak touch stimuli.

NMDA receptors perform a similar function, but, instead of acting on the presynaptic side of the cleft, the NMDA receptors act at the postsynaptic

terminal. The key biophysical property of these receptors is that they are both voltage-dependent and glutamate-dependent thereby allowing them to integrate signals and to evaluate associativity. These receptors work in the following way: At the membrane’s resting potential, the NMDA receptor ion channel is closed—it is physically blocked by an Mg2+ ion that sits inside the opening and blocks the pore. When the membrane is depolarized sufficiently the Mg2+ block is relieved, and once glutamate is bound, calcium ions can pass through the channel and enter the cell. The voltage dependence is the source of the receptor’s integrative property. No single synapse by itself can depolarize the postsynaptic membrane sufficiently to relive the magnesium block. Rather, many synaptic inputs acting in close spatial and temporal proximity to one another are needed to adequately depolarize the membrane. When depolarization and ligand binding occurs within an appropriate time window, the NMDA channel will open. Thus, the NMDA receptor provides synapses with a way of determining whether the associativity conditions for synaptic modification have been satisfied or not.

One of the ways that both the efficiency of synaptic transmission and the ability to vary this property can be controlled is through the balance between AMPA and NMDA receptors. At many postsynaptic terminals only NMDA receptors are active during development. These synapses remain silent at resting potentials because of the Mg2+ block, but are capable of transmitting signals when the membrane is depolarized by one means or another. In response to entry of calcium into the terminal through the NMDA receptors, AMPA receptors become active. The presence of active AMPA receptors allows the synapses to respond rapidly to glutamate and generate stronger responses at the postsynaptic terminal to the release of glutamate. AMPA receptors and AMPA receptor trafficking have a prominent role in learning and memory formation and for that reason are highlighted in Figure 21.5.

21.7 Hippocampal LTP Is an Experimental Model of Learning and Memory

The hippocampus is a horseshoe-shaped structure belonging to the limbic system. The hippocampus receives input from a number of sensory information processing areas. It, along with its surrounding structures, the entorhinal cortex, perirhinal cortex, and parahippocampal region, are believed to be the place where short term memories are stored and then shipped out to long term, more permanent memory storage locations through a process called memory consolidation. The hippocampal system is thought to be responsible for recalling spatial relationships between objects in the environment and for spatial navigation.

Hippocampal long-term potentiation (LTP) is a widely studied mammalian form of synaptic plasticity. This process is elicited in the laboratory

524 21. Learning and Memory

FIGURE 21.8. Hippocampal circuit in the rodent used in studies of LTP: The core circuit includes a set of one-way connections from the entorhinal cortex (EC) to the hippocampal dentate gyrus (DG), cornu ammonis 3 (CA3) region, CA1 region, subculum (Sub), and back to the EC and to other cortical regions. LTP is studied in the CA3 region, where there is a dense network of recurrent connections, and in the connections between the CA3 and CA1 neurons. Two sets of connections converge on the CA1 neurons—the Schaffer collaterals from the (ipsilateral) CA3 and the commissural fibers from the contralateral (contra) CA3. The names commonly given to the various axons are shown.

in brain slices prepared from portions of the rodent hippocampus called the CA1 and CA3 regions. The general architecture of the hippocampus showing the locations of the CA1 and CA3 regions is presented in Figure 21.8. Two sets of connections are noted in the figure—recurrent connections between cells in CA3, and connections from CA3 to CA1. In either circuit, when a tetanic stimulus (i.e., a train of high frequency pulses) is delivered to the presynaptic cell, or alternatively a series of pulses at low frequency is delivered and the event paired with a laboratory-supplied depolarization of the postsynaptic membrane, the efficiency of synaptic transmission is increased. The increase, or potentiation, of the synaptic efficiency can be generated in milliseconds, yet it lasts for minutes or even hours.

21.8 Initiation and Consolidation Phases of LTP

The initiation phase of LTP is followed by a consolidation phase involving gene expression. The transition from a short lived form of memory formation to a more permanent one lasting for days, weeks, and even years is through a process of memory consolidation. Gene expression is required during this phase of memory formation. The cyclic AMP (cAMP) signaling pathway is activated in which protein kinase A and downstream MAP kinases interact with cAMP response element-binding protein (CREB) transcription factors that bind to cAMP response elements (CREs) in promoters. This pathway can be activated in several ways. It can be activated through calcium regulation of adenylyl cyclases, and by neuromodulators

526 21. Learning and Memory

FIGURE 21.10. Downstream signaling events leading to consolidation of memories: Abbreviations—Immediate early gene (IEG); late gene (LG); cAMP response element-binding protein (CREB); p300/CREB-binding protein (CBP).

to promoter sites in the nucleus containing cAMP responsive elements (CREs). The kinases so involved include protein kinase A, protein kinase C, MAP kinases, and CaM kinase IV. Members of the CREB family of transcription factors bind to these sites. The CREB proteins remain inactive if not phosphorylated on Ser133. Phosphorylation on Ser133 by kinases such as PKA results in binding by p300/CREB-binding protein (CBP). As noted earlier, these are histone acetylases (HATs) that work together with the basal transcription machinery to initiate a program of transcription leading to formation of new synapses and morphological changes to old ones (Figure 21.10). As is the case for other transcriptional control points, a variety of factors acting positively and negatively regulate CREB-mediated transcription.

Transcription takes place in several stages, starting with immediate early genes and ending with late genes. For example, among the first genes to be transcribed in serotonin-mediated facilitation in Aplysia are those whose products extend to duration of the PKA signaling. This stage is followed by the transcription and synthesis of transcription factors such as C/EBP, which leads to the creation of new synaptic connections. This same overall set of signaling activities is observed in other forms of learning and memory, and in other test animals, so that there is a common family of mechanisms that extends across phyla from Aplysia to flies to mice.

21.10 Synapses Respond to Use by Strengthening and Weakening

In 1947, Donald Hebb formulated an associative rule governing usedependent changes in synaptic transmission. Hebb’s rule is couched in terms of cellular firing by a pair of cells (Figure 21.11). It says:When a when an axon of cell A is near enough to excite cell B and repeatedly and persistently takes

FIGURE 21.11. Hebb’s rule: (a) In response to the firing of an action potential by cell A, cell B fires an action potential. In this situation, the depolarization of the postsynaptic membrane contributes to the firing of an action potential by cell B. The strength of that synapse is increased. (b) Cell B does not fire an action potential when cell A does. The depolarization at the postsynaptic membrane is not effective in eliciting action potentials. In situations of this sort the strength of the synapse is reduced.

part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased. The rule in this form is incomplete. It does not state what happens when the postsynaptic cell fails to fire.To complete the rule one adds a subrule that says: When a presynaptic axon of cell A repeatedly and persistently fails to excite the postsynaptic cell B,while cell B is firing under the influence of other presynaptic axons, metabolic changes takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is decreased. In other words, synapses whose activation is strongly correlated with the firing of the postsynaptic cell are strengthened, while those synapses whose activation is poorly correlated with the firing of the postsynaptic cell, for example, by being silent while the cell fires, are reduced in efficiency.

According to the expanded form of Hebb’s rule two types of changes in the efficiency of synaptic transmission can occur: the synapse can be strengthened or it can be weakened. The efficiency of synaptic transmission increases when a synaptic connection is strengthened and decreases when the connection is weakened. Modifications in synaptic efficiency depend upon the timing of the action potentials in the preand postsynaptic cells. Repetitive preand postsynaptic firings occurring within 10 to 50 msec of one another are able to influence the efficiency of synaptic transmission. If cell A fires its action potential before cell B fires its action potential, then the synaptic connections between the two cells will be increased. If, on the other hand, cell B repeatedly fires before cell A, then the strength of the synapse will decrease. The situation is intrinsically asymmetric so that there is a preferred direction of information flow.

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21.11 Neurons Must Maintain Synaptic Homeostasis

If the efficiency of transmission increases in too many synapses of a neuron, excitation-induced toxic effects, such as excessive calcium levels, may set in. As a result the neuron may be harmed or even killed. In a typical Hebbian or associative process, each synapse acts in a fairly independent manner. There has to be some additional mechanism or constraint operating cell-wide that can throttle back runaway positive or negative changes in synaptic efficiency.

There are two, not necessarily distinct, ideas of how this may occur, both supported by a body of experimental data. One of these is the notion of a sliding modification threshold; the other is the idea of synaptic scaling.

In the sliding threshold model (Figure 21.12), a cell B possesses a threshold for synaptic modification. If the postsynaptic cell’s firing rate exceeds that threshold, paired preand postsynaptic firing will increase the efficiency of the synapse. If the postsynaptic cell’s firing rate falls below the threshold value, the same correlated activity will diminish the efficiency of that synapse. The effect is a dynamic one. Cell B will continually adjust its threshold according to the changes in the cell’s firing rate suitably averaged over time. The threshold will slide up or down as the firing rate increases or decreases. By moving up it becomes more difficult to produce further increases, and becomes easier to elicit decreases. Similarly, if the threshold goes down because of weak firing activity, it becomes easier to strengthen synapses and harder to weaken them.

In synaptic scaling, like the sliding threshold model, the total activity in the postsynaptic cell determines how that cell responds to neurotransmitter release. The firing rate reflects the sum of all synaptic activities in the postsynaptic cell. If the firing rate in the postsynaptic cell is already high all synaptic strengths will be scaled back, and conversely if the firing rate is too low the strength of each synapse will be scaled up. In mathematical terms, the effect is a multiplicative one. The strength at a particular synapse is proportional to the product of the firing rate and the synaptic strength at that

FIGURE 21.12. Sliding threshold model: Shown is a plot of the modification predicted by the sliding threshold model as a function of the total activity in the postsynaptic cell. Activity levels below that of the threshold produce long-term depression, or LTD (negative modification), while those that exceed the threshold lead to longterm potentiation, or LTP (positive modification). The threshold adapts to the overall activity in the postsynaptic cell averaged over a suitable time interval.

synapse. Since the scaling affects all synapses in the same way it may be best thought of as a form of cellular homeostasis operating in addition to Hebbian mechanisms.

21.12 Fear Circuits Detect and Respond to Danger

The forms of synaptic plasticity such as LTP that are observed in the hippocampus are not limited to that region. They are encountered in regions of the brain ranging from the visual cortex to the amygdala to regions of the brain involved in drug addiction. The amygdala is the central player in fear conditioning. In fear conditioning experiments using rats, tone signals are relayed through the thalamus to the auditory cortex and from the thalamus and auditory cortex to the amygdala. Foot shock signals are similarly relayed into the somatosensory cortex and from there to the amygdala.

The amygdala is partitioned into several regions. Some signals first enter the lateral nucleus (LN) of the amygdala and then pass to the basal nucleus (BN), while other signals enter the basal nucleus directly. These areas along with the accessory basal nucleus function as the central input unit of the amygdala that receive auditory, visual, somatosensory, and other forms of sensory information from the thalamic relay nuclei and higher sensory cortical areas. These nuclei also receive input information from the hippocampus of a contextual character, that is, environmental cues about, for example, place and space, associated with the experience. As shown in Figure 21.13 connections from the input unit are made to several regions. These include connections between the input unit and central nucleus, which sends projections to a number of brain stem regions and by that means controls the behavioral and physiological fear responses.

Fear conditioning produces synaptic modifications quite similar to hippocampal LTP. As is the case for LTP in hippocampal slices, LTP in the lateral and basal nuclei of the amygdala is dependent on postsynaptic depolarization. Calcium entry through NMDA receptors plays an important role, as does activation of and signaling by protein kinase A and MAP kinases. As was the case for LTP, there are several different forms ranging from short-term transitory changes to long lasting ones requiring CREB transcriptional activity and protein synthesis.

21.13 Areas of the Brain Relating to Drug Addiction

Drug addiction involves areas of the brain that shape mood and generate emotional responses. There are many different kinds of addictive drugs. Prominent examples of addictive drugs are alcohol, amphetamines, cocaine, morphine, and nicotine. Although the chemical makeup of each of these substances is different, all drugs target populations of neurons situated in

530 21. Learning and Memory

FIGURE 21.13. Circuitry involved in responses to fearful and dangerous sensory and contextual stimuli: Abbreviations: LN—Lateral nucleus of the amygdala; BN— Basal nucleus; ABN—Accessory basal nucleus; CN—Central nucleus. The hippocampal system responsible for relaying contextual information into the amygdala from sensory regions consists of the perirhinal cortex, entorhinal cortex, and hippocampus.

areas involved in mood and emotional responses and exhibit the following set of characteristics:

•the compulsion to take drugs,

•loss of control in limiting intake,

•entry into a negative emotional state when access to drugs is prevented, and

•relapses into addiction after periods of abstinence.

The changes induced by drug use are long-lived and involve changes at both the cellular and molecular levels. The cells and circuits that underlie drug addiction are located in several regions of the brain. They include cells that express acteylcholine receptors and communicate using dopamine (dopaminergic) and serotonin (serotonergic), and also include cells that release glutamate (glutamergic) or GABA (GABAergic). The corresponding brain regions are associated with reward, mood, arousal, and cognition. Long-lasting changes in synaptic transmission similar to those associated with learning and memory, but leading to addiction, take place in these brain regions.

The sites of action of addictive drugs such as cocaine, amphetamines, and nicotine lie deep in the brain, in regions associated with mood, emotions, and learning. As shown in Figure 21.14, drugs act on neurons in the upper part of the brain stem (ventral tegmental area and substantia nigra), the limbic system (amygdala and hippocampus), and the mesolimbic system

FIGURE 21.14. Side view of the brain showing the main sites of drug action: The regions of drug action—the ventral tegmental area, substantia nigra, nucleus accumbens, striatum, and prefrontal cortex—are shown in lighter and darker gray shades. Areas that form the nigrostriatal system are shown in light gray and those that the mesocorticolimbic system comprises are presented in dark gray.

(nucleus accumbens). The amygdala and hippocampus are ancient structures. The amygdala, which means “almond-shaped,” connects to hippocampus and prefrontal areas. Activity in this area of the brain is associated with strong emotions such as fear, aggression, and the fight or flight response. The hippocampus, meaning “seahorse-shaped,” is located just behind the amygdala. As discussed earlier in this chapter, this structure is associated with learning and memory, and especially with the transfer of information into memory.

21.14 Drug-Reward Circuits Mediate

Addictive Responses

Under the influence of drugs, neurons involved in mood, arousal, and cognition organize into drug-reward circuits. The core circuitry involved in drug addiction contains two systems—the nigrostriatal system and the mesocorticolimbic system. The substantia nigra (SN) is located in the upper part of the brain stem. It contains two groups of cell. One group, the dorsally located cells (the SN pars compacta, or SNc), uses dopamine to communicate with the corpus striatum. The other group, the ventrally positioned neurons (the SN pars reticulata, or SNr), utilizes GABA to communicate with the thalamus. The nigrostriatal system consists of neurons in the substantia nigra pars compacta and the corpus striatum, or striatum. The mesocorticolimbic system consists of cells in the ventral tegmental area (VTA) located near the SN, the nucleus accumbens (NAc) situated just above the